Testing of a four-channel Stokes polarimeter performance for intrusion detection in QKD systems

Abstract

The most mature quantum key distribution systems available on the market is the class of prepare-and-measure systems which are vulnerable to eavesdropping attacks. Therefore, monitoring optical properties of such a system should be enhanced. This paper presents a measurement methodology, characterisation, and testing of a four-channel Stokes polarimeter implemented into the send-and-return quantum key distribution system with faint pulses. The developed polarimetric setup is a complete amplitude-division Stokes polarimeter, the main objective of which is to define changes in the polarization state of light pulses. The measurement method is based on intensity measurements of four selected polarization components. The four-channel Stokes polarimeter was characterised and its performance was compared with a commercial instrument. At this stage of the polarimeter development, different signal processing paths were proposed. The influence of fibre infringement was observed as a rapidly changing polarisation state and, as a result, intrusion threshold levels were determined for the analysed signals for the quantum key distribution fibre-optic link tested.